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Direct Measurement of Neon Production Rates by (A,N) Reactions in Minerals

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Direct Measurement of Neon Production Rates by (A,N) Reactions in Minerals Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 148 (2015) 130–144 www.elsevier.com/locate/gca Direct measurement of neon production rates by (a,n) reactions in minerals a, a b Stephen E. Cox ⇑, Kenneth A. Farley , Daniele J. Cherniak a Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA b Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 2 April 2014; accepted in revised form 27 August 2014; available online 21 September 2014 Abstract The production of nucleogenic neon from alpha particle capture by 18O and 19F offers a potential chronometer sensitive to temperatures higher than the more widely used (U–Th)/He chronometer. The accuracy depends on the cross sections and the calculated stopping power for alpha particles in the mineral being studied. Published 18O(a,n)21Ne production rates are in poor agreement and were calculated from contradictory cross sections, and therefore demand experimental verification. Similarly, the stopping powers for alpha particles are calculated from SRIM (Stopping Range of Ions in Matter software) based on a limited experimental dataset. To address these issues we used a particle accelerator to implant alpha particles at precisely known energies 21 18 into slabs of synthetic quartz (SiO2) and barium tungstate (BaWO4) to measure Ne production from capture by O. Within experimental uncertainties the observed 21Ne production rates compare favorably to our predictions using published cross sec- tions and stopping powers, indicating that ages calculated using these quantities are accurate at the 3% level. In addition, we 22 21 measured the Ne/ Ne ratio and (U–Th)/He and (U–Th)/Ne ages of Durango fluorapatite, which is an important model sys- tem for this work because it contains both oxygen and fluorine. Finally, we present 21Ne/4He production rate ratios for a variety of minerals of geochemical interest along with software for calculating neon production rates and (U–Th)/Ne ages. Ó 2014 Elsevier Ltd. All rights reserved. 1. INTRODUCTION produced by these reactions is also readily apparent in crus- tal well gases (Kennedy et al., 1990). Yatsevich and Honda Alpha particles produced by the decay of uranium and (1997), who were primarily interested in the neon budget of thorium series nuclides interact with light elements in min- Earth’s mantle and atmosphere, proposed the use of nucle- erals to produce the three stable isotopes of neon. Nuclides ogenic neon as a chronometer. They established that the created by such secondary nuclear reactions are termed reaction 18O(a,n)21Ne dominates the nucleogenic produc- nucleogenic, and can be produced by alpha particle capture tion of neon in crust and mantle, and that the reaction and also by capture of neutrons emitted by alpha particle 24Mg(n,a)21Ne is insignificant in comparison. In addition, capture. Wetherill (1954) was the first to recognize produc- using published neutron capture cross section measure- tion of nucleogenic neon, from (a,n), (a,p), and (n,a) reac- ments they arrived at a mean bulk crustal production rate 21 4 8 tions in minerals containing uranium and thorium. Neon ratio Ne/ He = 4.5 10À , which is consistent with cur- Â rent estimates and measurements for U- and Th-bearing minerals, yet is much higher than earlier estimates (Kyser and Rison, 1982). Using similar data, Leya and Wieler ⇑ Corresponding author at: Caltech, MC 100-23, Pasadena, CA 21 22 91125, USA. Tel.: +1 (626) 395 2936. (1999) calculated the production rate of Ne and Ne in E-mail addresses: [email protected] (S.E. Cox), farley@gps. specific minerals of geochemical interest, such as apatite caltech.edu (K.A. Farley), [email protected] (D.J. Cherniak). and zircon. Gautheron et al. (2006) modeled the production http://dx.doi.org/10.1016/j.gca.2014.08.036 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved. S.E. Cox et al. / Geochimica et Cosmochimica Acta 148 (2015) 130–144 131 rate of 18O(a,n)21Ne on the basis of a different analysis of (Yatsevich and Honda, 1997; Leya and Wieler, 1999; the published cross section data and arrived at production Gautheron et al., 2006). However, a known rate of neon rates that differed by up to 10% from those of Leya and production is critical for any chronometric application, Wieler (1999). They also presented (U–Th)/Ne age determi- and existing estimates based on neutron production mea- nations on several apatite and zircon samples using these surements are not in sufficient agreement for high-precision production rates and considered the consequences for dat- geochronology (Leya and Wieler, 1999; Gautheron et al., ing of the spatial separation of parent nuclides from both 2006). In this paper we take a different approach than pre- helium and neon daughter products due to alpha ejection vious workers to establishing 21Ne production rates. We and the energy-dependence of the neon production rate. measure the neon itself in target phases exposed to energetic The (U–Th)/Ne geochronometer is potentially useful in alpha particles of precisely known energy. This work is rapidly cooled samples and as a chronometer of the mini- analogous to the oxygen gas target experiments of mum age of mineral formation. In the former case, one sim- Hu¨nemohr (1989) but undertaken on solid phases. We also ply assumes that the rock cooled so quickly that diffusive loss present new high precision U–Th–He–Ne data on Durango of neon has been negligible for its entire history, and in the apatite, which confirm the validity of our 21Ne results in a latter case, one acknowledges that diffusive loss may have natural system and also provide a production rate estimate occurred and accounts for this possibility in the geologic for the 22Ne reaction from alpha capture by 19F. interpretation. For example, Farley and Flowers (2012) For (U–Th)/Ne chronometry, we are primarily inter- applied (U–Th)/Ne dates as a minimum age for the forma- ested in the two reactions 18O(a,n)21Ne and 19F(a,n)22- tion of a hematite specimen from the Grand Canyon. Na(b+)22Ne (simplified as 19F(a,n)22Ne hereafter due to As discussed by Gautheron et al. (2006), (U–Th)/Ne the insignificance of the 2.6 year 22Na decay over geologic ages can also be used for thermochronometry. Ideally a time). These reactions occur at a rate proportional to the thorough understanding of the temperature dependence of number of alpha particles emitted and to the concentration neon diffusion supports such an application. However, of the target nuclide (18O or 19F) in the mineral. Further- because laboratory diffusivity measurements are usually more, the rate of these reactions also depends on the cross made under environmental conditions (temperature, pres- section of the target nuclide for alpha particles and the sure, chemical activity) much different from the natural set- chemical composition of the host mineral. Below, we ting, care is required in interpreting the laboratory data. describe the physical controls on this reaction and how they This is especially true for some phases of interest for are measured or calculated in order to arrive at a nucleo- (U–Th)/Ne dating, such as apatite, because phase modifica- genic neon production rate. tion via volatile loss in a high temperature vacuum chamber Neon isotope production from (n,a) reactions on Mg is possible (Nadeau et al., 1999). As an alternative approach, can be safely ignored for minerals that do not contain large approximate closure temperatures for neon can be obtained amounts (wt%) of Mg. For example, O is about three times by comparison of neon ages to those of other thermochro- as productive of neon as Mg, so these production pathways nometers in the same rock (Gautheron et al., 2006). only become significant at the 1% level for Mg/O > 0.03 (in the case of 21Ne). Magnesium is even less productive of 1.1. The production of neon by uranium and thorium decay neon than F, so these production pathways become signif- icant at the 1% level for Mg/F > 0.07 (in the case of 22Ne). In the (U–Th)/He system, multiple parents (uranium- In minerals that include magnesium as a stoichiometric 238, uranium-235, and thorium-232) decay through chains constituent, these reactions must be considered in the neon in which they emit a series of alpha particles with discrete budget. energies. Samarium-147 is also an alpha emitter, but The rate of a given nuclear reaction depends on the because it produces just one alpha particle and because of nuclear properties of the target nuclide and the energy of its very low energy, this nuclide contributes little to nucleo- the incident particle, and is characterized by the nuclear genic production. The vast majority of alpha particles come cross section parameter (r(E)). The cross section is an areal to rest as 4He atoms, but a small number (fewer than one in representation of the probability of reaction in a pure sam- ten million) react with other elements in a mineral to form ple of the target nuclide and is frequently described in units 24 2 nuclear reaction products. A particularly important reac- of millibarns (1 barn = 10À cm ). A larger cross section tion is 18O(a,n)21Ne, which produces measurable neon in means that the reaction is proportionally more likely to many oxide and silicate minerals (Yatsevich and Honda, occur when a single particle interacts with the target, or 1997; Leya and Wieler, 1999; Gautheron et al., 2006; proportionally more frequent when a large number of par- Farley and Flowers, 2012).
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